The present invention relates to the liquid washing of gases containing sulfur dioxide and, more particularly, to a process and apparatus for such washing which results in minimum scaling in the washing system.
The combustion of fossil fuels such as coal and oil results in the production of sulfur dioxide from oxidation of the naturally occurring sulfur in the fossil fuel. Sulfur dioxide is a particularly noxious air pollutant and, accordingly, power generating plants and other large users of fossil fuels are constantly looking for methods of removing the sulfur dioxide from the flue gases. One method which has proven quite effective involves washing the sulfur dioxide laden waste gases with a slurry of an alkaline metal carbonate, oxide, or hydroxide such as a lime or limestone slurry or a magnesium oxide slurry. However, the use of such a wet scrubbing system presents certain problems, a major one being scaling of the equipment surfaces with the metal sulfate and/or sulfite such as gypsum (calcium sulfate) and/or calcium sulfite. Indeed, in certain cases the scaling can become so severe on the scrubber surfaces and the attendant equipment that shutdown of the system is necessitated because of curtailed gas or liquid flow. Another problem connected with such wet scrubbing processes is that the solid sulfite and/or sulfate produced in the scrubber has a particle size which makes it difficult to dewater or otherwise process.
The reaction of sulphur dioxide with lime, limestone or similar substances in a wet scrubbing operation results, in part, in the formation of crystalline metal sulfite, e.g. calcium sulfite, along with dissolved sulfite. Since oxygen is generally present in any such wet scrubbing system, there is resultant oxidation of dissolved sulfite, part of which ends up as crystalline, substantially insoluble metal sulfate, e.g. calcium sulfate, a species which can also cause scaling.
It is well known that both calcium sulfate and calcium sulfite will form supersaturated solutions. It is also known that in highly supersaturated solutions, spontaneous nucleation takes place, which, while ultimately leading to the formation of crystalline precipitants of the sulfite and sulfate facilitating their removal from the system, also leads to scaling i.e. plating out on the equipment surfaces. In other words, if the driving force for crystallization becomes high enough in a highly supersaturated solution of sulfite/sulfate, the capacity of the circulating medium to hold the sulfite or sulfate in a harmless form, i.e. in solution or as a precipitate on circulating solids, is exceeded and crystallization on the scrubber system surfaces occurs.
Numerous methods have been proposed to cope with the problem of scaling in lime/limestone wet scrubbing systems. The problem has been treated extensively in the literature. See, for example, Slack A. V., REMOVAL OF SULFUR DIOXIDE FROM STACK GASES BY SCRUBBING WITH LIMESTONE SLURRY: OPERATIONAL ASPECTS OF THE SCALING PROBLEM, Prepared for Presentation at the Second International Lime/Limestone Wet Scrubbing Symposium Sponsored by the EPA, New Orleans, Louisiana, Nov. 8-12, 1971. In addition, U.S. Pat. Nos. 2,090,142 and 2,090,143 to Nonhebel et al describe processes for preventing scale formation in the scrubbing processes under consideration. Basically, the processes described in those patents involve subjecting the scrubbing medium leaving the scrubber to a delay time long enough to enable desupersaturation of the liquor by permitting substantially complete crystallization of the reaction products, and by controlling the scrubbing medium flow rate such that the extent of supersaturation existing in the scrubbing medium leaving the scrubber does not exceed certain critical values. Not only do the processes described in the Nonhebel et al patents require extremely large hold tanks to achieve the requisite delay time but, moreover, the surfaces of the scrubbing equipment must be constructed of certain types of materials.
It is well known that desupersaturation of a solution can be accomplished by contact with crystals of the salt with respect to which the solution is supersaturated. However, the mere presence in a lime/limestone scrubbing system of high concentration of solid sulfite/sulfate in the circulating scrubbing medium to thereby provide a surface on which dissolved sulfite and sulfate can preferentially precipitate has not been proven to be satisfactory in terms of preventing scaling. Indeed, it has been found that in scrubbing systems of the type under consideration, supersaturation and resultant scaling can occur on the scrubbing surfaces even when the concentration of suspended calcium sulfite and sulfate are greater than 5% each. Thus, while it has been suggested that seeding will effect desupersaturation and minimize scaling, the methods heretofore employed have not been successful.
The present invention is based upon the observation that the controlled introduction of seed crystals into the system in such a fashion that process particle balance is maintained will result in positive prevention of scale and control of the solid waste particle size distribution. This controlled introduction of seed crystals is accomplished by a process and means other than uncontrolled spontaneous nucleation within the system, i.e. uncontrolled chemical precipitation, or by indiscriminate seed crystal generation by a mechanical means within the system. In the latter respect, U.S. Pat. No. 3,708,266 to Gustavsson discloses an apparatus for removing sulfur dioxide from flue gases in which the total circulating slurried absorbant is passed through a grinding mill which removes the sulfite/sulfate layer from the surface of the active carbonate particles, the rejuvenated carbonate particles being returned to the scrubber. The Gustavsson system can, for practical purposes, be considered uncontrolled seed crystal generation much like the uncontrolled chemical precipitation which occurs via spontaneous nucleation.